1. Field of the Invention
The present invention relates to a vehicle motion control system for controlling vehicle motion, and more particularly to the control system for restraining an excessive oversteer and excessive understeer which will occur during, for example, cornering, by applying a braking force to each wheel of the vehicle irrespective of depression of a brake pedal.
2. Description of the Related Arts
Recently, a vehicle is provided with a braking force control system for controlling the braking force applied to the vehicle to perform an anti-skid control, a traction control, a front-rear braking force distribution control, and etc. In the U.S. Pat. No. 4,898,431, for example, an apparatus for controlling vehicle motion through the use of a brake controlling system which compensates for the influence of lateral forces on the vehicle. The apparatus is constituted so as to control the braking force applied to the vehicle by the braking force control system in response to a comparison of a desired yaw rate with an actual yaw rate, thereby to improve the vehicle stability during the course of vehicle motion such as cornering.
In general, "oversteer" and "understeer" are used to indicate a vehicle steering characteristic. When the oversteer is excessive during vehicle motion such as cornering, the rear wheels of the vehicle have a tendency to slip excessively in the lateral direction to cause a decrease in turning radius of the vehicle. The oversteer occurs when a cornering force CFf of the front wheels largely exceeds a cornering force CFr of the rear wheels (i.e., CFf&gt;&gt;CFr). As shown in FIG. 14, when a vehicle VL is undergoing a cornering maneuver along a curve of a turning radius R, for example, a lateral acceleration Gy which is normal to the vehicle's path of motion is calculated in accordance with an equation of Gy=V.sup.2 /R, where "V" corresponds to a vehicle speed, and a total CFo of the cornering force is calculated in accordance with the following equation: EQU CFo=.SIGMA.CF=m.multidot.Gy
where "m" corresponds to a mass of the vehicle VL. Accordingly, in the case where the sum of the cornering force CFf and the cornering force CFr is larger than the total cornering force CFo for the vehicle's cornering maneuver along the curve of the turning radius R (i.e., CFo&lt;CFf+CFr), and the cornering force CFf of the front wheels largely exceeds the cornering force CFr of the rear wheels (i.e., CFf&gt;&gt;CFr), i.e., the oversteer is excessive, then the vehicle VL will be forced to turn in a direction toward the inside of the curve in the vehicle's path to cause a reduce in turning radius of the vehicle VL as shown in FIG. 14.
When the understeer is excessive during cornering, the lateral slip of the vehicle will be increased, the vehicle VL will be forced to turn in a direction toward the outside of the curve in the vehicle's path to cause an increase in turning radius of the vehicle VL as shown in FIG. 15. Thus, the excessive understeer occurs when the cornering force CFf of the front wheels is almost equal to the cornering force CFr of the rear wheels so as to be balanced with each other, or the latter is slightly larger than the former (i.e., CFf&lt;CFr), and when the sum of the cornering force CFf and the cornering force CFr is smaller than the total cornering force CFo which is required for the vehicle's cornering maneuver along the curve of turning radius R (i.e., CFo&gt;CFf+CFr), then the vehicle VL will be forced to turn in the direction toward the outside of the curve in the vehicle's path thereby to increase the turning radius R.
The excessive oversteer is determined on the basis of a vehicle side slip angle or vehicle slip angle .beta. and a vehicle slip angular velocity D.beta., for example. When it is determined that the excessive oversteer occurs during cornering, a braking force will be applied to a front wheel located on the outside of the curve in the vehicle's path, for example, to produce a moment for forcing the vehicle to turn in the direction toward the outside of the curve, i.e., an outwardly oriented moment, in accordance with an oversteer restraining control, which may be called as a vehicle stability control. On the other hand, the excessive understeer is determined on the basis of a difference between a desired lateral acceleration and an actual lateral acceleration, or a difference between a desired yaw rate and an actual yaw rate, for example. When it is determined that the excessive understeer occurs while a rear-drive vehicle is undergoing a cornering maneuver, for example, the braking force will be applied to a front wheel located on the outside of the curve and applied to both of the rear wheels to produce a moment for forcing the vehicle to turn in the direction toward the inside of the curve, i.e., an inwardly oriented moment, in accordance with an understeer restraining control, which may be called as a course trace performance control. The above-described oversteer restraining control and understeer restraining control as a whole may be called as a steering control by braking.
In Japanese Patent Laid-open Publication No. 7-125625, there is disclosed an apparatus for correcting an abnormal motion of a vehicle by applying a braking force to each wheel. With respect to a prior apparatus, it is raised in the publication such a problem that a desired slip rate is determined only on the basis of a motion of the vehicle, irrespective of a driving condition of the vehicle driven in response to depression of an accelerator pedal, so that a vehicle driver's intention is never reflected during the correcting operation of the vehicle. According to the apparatus proposed in the publication, therefore, the desired slip rate is corrected in accordance with the braking force applied to a driven wheel, thereby to control the braking force smoothly and reflect the driver's intention.
According to the apparatus disclosed in the Patent Laid-open Publication No. 7-125625, however, a desired slip rate provided for a non-driven wheel is not corrected, while the desired slip rate provided for the driven wheel is corrected. Therefore, in the case where an engine brake is exerted on a vehicle during the above-described braking control thereof, if an actual slip rate of the driven wheel exceeds the desired slip rate, a lateral force of the driven wheel will be reduced to be lower than the desired value. As a result, the above-described steering control by braking might be deteriorated.
Since the above-described apparatus is aimed to reflect the driver's intention to accelerate the vehicle, it might be sufficient to correct the desired slip rate only for the driven wheel. However, when the driver release the accelerator pedal, the engine brake is caused to apply to the driven wheel the braking force which is to be made relatively large in a specific vehicle condition. Accordingly, a braking force exceeding the braking force which is necessary for the steering control by braking may be applied to the driven wheel, so that the lateral force to the driven wheel might be reduced. If the braking force caused by the engine brake is smaller than the desired braking force, the steering control by braking will not be deteriorated. However, the braking force may be increased by the engine brake to exceed the desired braking force, even when it is necessary to reduce the braking force during the steering control by braking. As a result, an excessive oversteer might be caused with respect to a rear-drive vehicle, and an excessive understeer might be caused with respect to a front-drive vehicle.
In such a condition that the engine brake is exerted on the vehicle, it is impossible to reduce the braking force applied to the driven wheel, which exceeds the desired braking force, but it is possible to increase the braking force applied to the non-driven wheel so as to balance the braking force between the front section and rear section of the vehicle, whereby the necessary lateral force can be obtained. In other words, if the braking force applied to the nondriven wheel is increased to balance with the braking force applied to the driven wheel so as to control a braking force distribution between the driven wheel and non-driven wheel to be a desired distribution for the steering control by braking, the necessary lateral force can be obtained. In stead, a driving force applied to the driven wheel can be reduced. In this case, however, it is necessary to provide another apparatus for controlling the driving force, so that the apparatus will become large in scale and high in cost.